The field of this invention relates generally to gas turbine engines and more particularly to the repair of turbine nozzle segments used in turbine engines. A gas turbine engine includes a compressor that provides pressurized air to a combustor. Air is mixed with fuel in the combustor and ignited, generating hot combustion gases. These gases flow downstream to a turbine section where energy is extracted to power the compressor and provide useful work, such as powering an aircraft. Aircraft engines typically include stationary turbine nozzles that enhance engine performance by directing gas flow within the turbine section. In multi-stage turbine sections, turbine nozzles are placed at the entrance of each turbine stage to channel combustion gases into the turbine rotor located downstream of the nozzles. Turbine nozzles are typically circumferentially segmented, with each nozzle segment having one or more vanes positioned between inner and outer bands that define the radial flowpath boundaries for the hot combustion gases flowing through the nozzles. These nozzle segments are mounted to the engine casing to form an annular array with the vanes extending radially between the rotor blades of adjacent turbine stages.
Various approaches have been proposed for manufacturing and repairing nozzle segments. In one approach, the nozzle segment is a multi-piece assembly comprising an inner band, an outer band and one or more vanes, each of which is individually cast. Both the inner and outer bands are provided with slots into which the ends of the vanes are brazed in place to form the nozzle segment assembly. Another approach is to integrally cast the nozzle segment with the vanes, inner band and outer band formed together as a one-piece casting.
Nozzle segments are exposed during operation to a high temperature, corrosive gas stream that limits their effective service life. Accordingly, nozzle segments are typically fabricated from high temperature cobalt or nickel-based superalloys coated with corrosion and/or heat resistant materials. The nozzle segments are ordinarily cooled internally with cooling air extracted from the compressor to prolong service life. Even with such efforts, portions of the nozzle segments, particularly the vanes, can become cracked, corroded, and otherwise damaged such that the nozzle segments must be either repaired or replaced to maintain safe, efficient engine operation. Conventional repair methods involve weld or alloy repair of the damaged regions of the nozzle segment. Because nozzle segments are complex in design, are made of relatively expensive materials, and are expensive to manufacture, it is desirable to repair them whenever possible, rather than replace them with new nozzle segments.
U.S. Pat. No. 6,416,278 (“'278 patent”) refers to nozzle segments that were damaged during engine operation to the point where they could not be repaired by then known repair processes. The '278 patent discloses a method for repairing a turbine nozzle segment that includes separating the inner band from the nozzle segment, and joining the inner band to a newly manufactured replacement casting having an outer band and at least one vane. The replacement casting includes a mounting platform formed on one end of the vane and a boss formed on the mounting platform. A collar is joined to the inner band and has a slot formed therein. The boss is then inserted into the slot, and the mounting platform is received in a recess formed in the inner band. Joinder of the collar to the nozzle segment is completed by joining the boss to the collar and the mounting platform to the inner band. However, the size of the nozzle segment is integral to the use of the repair method disclosed in the '278 patent. Stage 2 HPT nozzle segments of the type disclosed in this application are of a size rendering them incapable of accepting a collar during the re-application of the airfoils, as described in the '278 patent. The repair method disclosed in the '278 patent is also limited by the length and width of any crack, the degree of erosion of the parent material, and the wall thickness of the nozzle segment.
In some instances replacement of the turbine nozzle vane airfoil is necessary, as opposed to conventional repair. At present, this is accomplished by discarding the entire nozzle segment and replacing it with a completely new replacement part. This involves discarding components of the nozzle segment with remaining useful life along with worn or damaged parts. In particular, the honeycomb backing strip of a nozzle segment often has remaining life even though the rest of the nozzle segment is damaged or worn beyond repair. The honeycomb backing strip is fabricated with very expensive alloys, so that the ability to reuse this component would result in substantial savings. Therefore, there is a need for a method of repairing a turbine engine nozzle segment that permits reuse of honeycomb backing strips removed from nozzle segments that are otherwise damaged or worn beyond repair, and must therefore be discarded and replaced. There is also a need for a method of repairing a turbine engine nozzle segment that permits the worn parts of the nozzle segment to be replaced with parts fabricated of enhanced materials in order to achieve greater part longevity and operational efficiency.